1. Field of the Invention
The present invention pertains to passive and semi-passive radio-frequency identification systems and, more particularly, to a microprocessor-based RFID system for monitoring and controlling remote devices.
2. Description of the Related Art
Radio-frequency tags are becoming common for remote inventory of items that are associated with the tags. Typically, the tags have a memory containing information about the respective items. The stored information is communicated to a remote reader via continuous wave backscatter modulation in response to an interrogation signal.
As shown in FIG. 1, a basic RFID system 10 includes two components: an interrogator or reader 12, and a transponder (commonly called an RF tag) 14. The interrogator 12 and RF tag 14 include respective antennas 16, 18. In operation, the interrogator 12 transmits through its antenna 16 a radio-frequency interrogation signal 20 to the antenna 18 of the RF tag 14. In response to receiving the interrogation signal 20, the RF tag 14 produces an backscatter modulated response signal 22 that is reflected back to the interrogator 12 through the tag antenna 18. This process is known as modulated backscatter.
The conventional RF tag 14 includes an amplitude modulator 24 with a switch 26, such as a MOS transistor, connected between the tag antenna 18 and ground. When the RF tag 14 is activated by the interrogation signal 20, a driver (not shown) creates a modulating signal 28 based on an information code, typically an identification code, stored in a non-volatile memory (not shown) of the RF tag 14. The modulating signal 28 is applied to a control terminal of the switch 26, which causes the switch 26 to alternately open and close. When the switch 26 is open, the tag antenna 18 reflects a portion of the interrogation signal 20 back to the interrogator 12 with one amplitude and phase as a portion 28 of the response signal 22. When the switch 26 is closed, the tag antenna reflects a second amplitude phase. In other words, the interrogation signal 20 is amplitude-modulated to produce the response signal 22 by alternately reflecting and absorbing at a different amplitude and phase the interrogation signal 20 according to the modulating signal 28, which is characteristic of the stored information code. Upon receiving the response signal 22, the interrogator 12 demodulates the response signal 22 to decode the information code represented by the response signal.
The substantial advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. The interrogator 12 emits the interrogation signal 20 with a range from one inch to one hundred feet or more, depending upon its power output and the radio-frequency used. Tags can be read through a variety of parameters, such as odors, or substances such as fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RF tags can also be read at remarkable speeds, in most cases responding in less than one hundred milliseconds.
A typical RF tag system 10 will contain a number of RF tags 14 and one or more interrogators 12. The three main categories of RF tags are beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in fundamentally different ways.
The beam-powered RF tag is often referred to as a passive device because it derives the energy needed for its operation from the interrogation signal beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power just for auxiliary circuit. Finally, in the active RF tag, a transmitter is used to create its own radio-frequency energy powered by the battery.
The range of communication for such tags varies according to the transmission power of the interrogator 12 and the RF tag 14. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach up to 200 meters in range, depending on the frequency and environmental characteristics.
An example of an active RFID system is found in U.S. Pat. No. 6,061,614 for an electronic tag including RF modem for monitoring motor vehicle performance. Here, a tag has a transceiver for transmitting data to and receiving data from a host, and the tag is coupled to a bus in the motor vehicle for receiving data from various systems in the motor vehicle regarding motor vehicle performance. In this use of radio-frequency technology, the tag is powered by the electrical system of the motor vehicle because of the substantial power requirements for operating the various components of the tag.
There is a need for a tag that can operate on power from the received radio-frequency signal only and that is of a lightweight, small size, and low cost to manufacture, and which can utilize input and output signals for onboard or remote components, circuits, or devices.